The present invention relates generally to the field of anti-counterfeiting and authentication methods and devices and, more particularly, to methods, security devices and apparatuses for authenticating documents and valuable products by band moiré patterns.
Counterfeiting of documents such as banknotes is becoming now more than ever a serious problem, due to the availability of high-quality and low-priced color photocopiers and desk-top publishing systems. The same is also true for other valuable products such as CDs, DVDs, software packages, medical drugs, watches, etc., that are often marketed in easy to falsify packages.
The present invention is concerned with providing a novel security element and authentication means offering enhanced security for devices needing to be protected against counterfeits, such as banknotes, checks, credit cards, identity cards, travel documents, valuable business documents, industrial packages or any other valuable products.
The theory on which the present invention relies will be published at the beginning of August 2004, as a scientific contribution: “Band Moiré Images”, by R: D. Hersch and S. Chosson, SIGGRAPH'2004, ACM Computer Graphics Proceedings, Vol. 23, No. 3.
Various sophisticated means have been introduced in the prior art for counterfeit prevention and for authentication of documents or valuable products. Some of these means are clearly visible to the naked eye and are intended for the general public, while other means are hidden and only detectable by the competent authorities, or by automatic devices. Some of the already used anti-counterfeit and authentication means include the use of special paper, special inks, watermarks, micro-letters, security threads, holograms, etc. Nevertheless, there is still an urgent need to introduce further security elements, which do not considerably increase the cost of the produced documents or goods.
Moiré effects have already been used in prior art for the authentication of documents. For example, United Kingdom Pat. No. 1,138,011 (Canadian Bank Note Company) discloses a method which relates to printing on the original document special elements which, when counterfeited by means of halftone reproduction, show a moiré pattern of high contrast. Similar methods are also applied to the prevention of digital photocopying or digital scanning of documents (for example, U.S. Pat. No. 5,018,767, inventor Wicker). In all these cases, the presence of moiré patterns indicates that the document in question is counterfeit.
Other prior art methods, on the contrary, take advantage of the intentional generation of a moiré pattern whose existence, and whose precise shape, are used as a means of authenticating the document. One known method in which a moiré effect is used to make visible a hidden pattern image encoded within a document (see background of U.S. Pat. No. 5,396,559 to McGrew, background of U.S. Pat. No. 5,901,484 to Seder, U.S. Pat. No. 5,708,717 to Alasia and U.S. Pat. No. 5,999,280 to Huang) is based on the physical presence of that image on the document as a latent image, using the technique known as “phase modulation”. In this technique, a line grating or a random screen of dots is printed on the document, but within the pre-defined borders of the latent image on the document the same line grating (or respectively, the same random dot-screen) is printed at a different phase, or possibly at a different orientation. For a layman, the latent image thus printed on the document is difficult to distinguish from its background; but when a revealing layer comprising an identical, but unmodulated, line grating or grating of lenticular lenses (respectively, random dot-screen) is superposed on the document, thereby generating a moiré effect, the latent image pre-designed on the document becomes clearly visible, since within its pre-defined borders the moiré effect appears in a different phase than in the background. Such a latent image may be recovered, since it is physically present on the document and only filled by lines at different phases or by a different texture. A second limitation of this technique resides in the fact that there is no enlargement effect: the pattern image revealed by the superposition of the base layer and of the revealing transparency has the same size as the latent pattern image. It should be stressed the disclosed band moire image synthesizing methods completely differ from the above mentioned technique of phase modulation since no latent image is present when generating a band moire image and since the band moiré image pattern shapes resulting from the superposition of a base band grating and a revealing line grating are a transformation of the original pattern shapes embedded within the base band grating.
This transformation comprises always an enlargement, and possibly a rotation, a shearing, a mirroring, and/or a bending transformation. In addition, in the present invention, base band grating and revealing line grating layers can be created where translating respectively rotating the revealing layer on top of the base layer yields a displacement of the band moiré image patterns. Phase based modulation techniques allowing to hide latent images within a base layer are not capable of smoothly displacing and possibly transforming the revealed latent image when moving the revealing layer on top of the base layer. For example, they are unable to create a continuous displacement of the band moiré image patterns, such as for example the band moiré image patterns moving towards the center of a circular band moiré image layout. A further means of distinguishing phase modulation techniques from band moirés consists in verifying, once the revealing line grating is laid out on top of the base layer, if respectively a moiré pattern is produced by sampling only a single instance (i.e. one latent pattern image) or multiple instances of a base layer pattern (i.e. multiple base bands incorporating each one an instance of the base band pattern).
U.S. Pat. No. 5,999,280, Holographic Anti-Imitation Method and Device for preventing unauthorized reproduction, inventor P. P. Huang, issued Dec. 7, 1999, discloses a holographic anti-imitation method and device where the superposition of a viewing device on top of a hidden pattern merged on a background pattern allows to visualize that hidden pattern. This disclosure relies on a technique similar to the phase modulation technique presented in the background section of U.S. Pat. No. 5,396,559 to McGrew, implemented on a holographic device. In contrast to U.S. Pat. No. 5,999,280, our invention relies on a completely different principle: several instances of the base band patterns are sampled and produce band moire image patterns which are enlarged and transformed instances of these base band patterns. Furthermore, our invention allows to generate dynamic band moire images, i.e. animations with dynamically behaving band moire image pattern shapes, which are impossible to achieve with U.S. Pat. No. 5,999,280.
In U.S. Pat. No. 5,712,731 (Drinkwater et al.) a moiré based method is disclosed which relies on a periodic 2D array of microlenses. However, this last disclosure has the disadvantage of being limited only to the case where the superposed revealing structure is a microlens array and the periodic structure on the document is a constant 2D dot-screen with identical dot-shapes replicated horizontally and vertically. Thus, in contrast to the present invention, that invention excludes the use of gratings of lines as the revealing layer, both imaged on a transparent support (e.g. film) or as a grating of cylindric microlenses.
Other moiré based methods disclosed by Amidror and Hersch in U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638 rely on the superposition of arrays of screen dots which yields a moiré intensity profile indicating the authenticity of the document. These inventions are based on specially designed 2D periodic structures, such as dot-screens (including variable intensity dot-screens such as those used in real, gray level or color halftoned images), pinhole-screens, or microlens arrays, which generate in their superposition periodic moiré intensity profiles of chosen colors and shapes (typographic characters, digits, the country emblem, etc.) whose size, location and orientation gradually vary as the superposed layers are rotated or shifted on top of each other. In a third invention, U.S. patent application Ser. No. 09/902,445, Amidror and Hersch disclose new methods improving their previously disclosed methods mentioned above. These new improvements make use of the theory developed in the paper “Fourier-based analysis and synthesis of moirés in the superposition of geometrically transformed periodic structures” by I. Amidror and R. D. Hersch, Journal of the Optical Society of America A, Vol. 15, 1998, pp. 1100-1113 (hereinafter, “[Amidror98]”), and in the book “The Theory of the Moiré Phenomenon” by I. Amidror, Kluwer, 2000. According to this theory, said invention discloses how it is possible to synthesize aperiodic, geometrically transformed dot screens which in spite of being aperiodic in themselves, still generate, when they are superposed on top of one another, periodic moiré intensity profiles with undistorted elements, just like in the periodic cases disclosed by Hersch and Amidror in their previous U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638. U.S. patent application Ser. No. 09/902,445 further disclosed how cases which do not yield periodic moirés can still be advantageously used for anticounterfeiting and authentication of documents and valuable products. In U.S. patent application Ser. No. 10/183,550 “Authentication with build-in encryption by using moiré intensity profiles between random layers”, inventor Amidror discloses how a moiré intensity profile is generated by the superposition of two specially designed random or pseudo-random dot screens. An advantage of that invention relies in its intrinsic encryption system offered by the random number generator used for synthesizing the specially designed random dot screens.
However, the disclosures above made by inventors Hersch and Amidror (U.S. Pat. No. 6,249,588, U.S. Pat. No. 5,995,638. U.S. patent application Ser. No. 09/902,445) or Amidror (U.S. application Ser. No. 10/183,550) making use of the moiré intensity profile to authenticate documents have two limitations. The first limitation is due to the fact that the revealing layer is made of dot screens, i.e. of a set (2D array) of tiny dots laid out on a 2D surface. When dot screens are embodied by an opaque layer with tiny transparent dots or holes (e.g. a film with small transparent dots), only a limited amount of light is able to traverse the dot screen and the resulting moiré intensity profile is not easily visible. In these inventions, to make the moiré intensity profile clearly visible, one needs to work in transparent mode; both the revealing and the base layers need to be placed in front of a light source and the base layer should be preferably printed on a partly transparent support. In reflective mode, one needs to use a microlens array as master screen which, thanks to the light focussing capabilities of the lenses, make the moiré intensity profile clearly visible. The second limitation is due to the fact that the base layer is made of a two-dimensional array of similar dots (dot screen) where each dot has a very limited space within which only a few tiny shapes such as a few typographic characters or a single logo must be placed. This space is limited by the 2D frequency of the dot screen, i.e. by its two period vectors. The higher the 2D frequency, the less space there is for placing the tiny shapes which, when superposed with a 2D circular dot screen as revealing layer, produce as 2D moiré an enlargement of these tiny shapes.
In U.S. patent application Ser. No. 10/270,546 (filed 16 Oct. 2002, “Authentication of documents and articles by moiré patterns”, inventors Hersch and Chosson), a significant improvement was made by the discovery that a rectilinear base band grating incorporating original shapes superposed with a revealing straight line grating yields rectilinear moiré bands comprising moiré shapes which are a linear transformation of the original shapes incorporated within the base band grating. These moiré bands form a band moiré image. Since band moiré have a much better light efficiency than moiré intensity profiles relying on dots screens, band moiré images can be advantageously used in all case where the previous disclosures relying on 2D screens fail to show strong enough moiré patterns. In particular, the base band grating incorporating the original pattern shapes may be printed on a reflective support and the revealing line screen may simply be a film with thin transparent lines. Due to the high light efficiency of the revealing line screen, the band moiré patterns representing the transformed original band patterns are clearly revealed. A further advantage of band moiré images resides in the fact that it may comprise a large number of patterns, for example one or several words, one or several sophisticated logos, one or several symbols, and one or several signs.
U.S. patent application Ser. No. 10/270,546 (Hersch and Chosson), describes the layout of rectilinear band moiré images, when the layouts of base layer and the revealing layer are known. However it does not tell in which direction and at which speed the moiré shape moves when translating the rectilinear revealing layer on top of the rectilinear base layer. Furthermore, since it does not disclose a model for predicting the layout of the moiré image that can be produced when superposing a curvilinear base layer and a curvilinear revealing layer, band moirés image relying on curvilinear base or revealing layers need to be generated by a trial and error procedure. One tries first to generate examples of curvilinear line moirés produced by the superposition of line grating (according to the theory describing prior art line grating, see the article by I. Amidror and R. D. Hersch, Fourier-based analysis and synthesis of moirés in the superposition of geometrically transformed periodic structures, Journal of the Optical Society of America A, Vol. 15, 1998; pp. 1100-1113 or the book of I. Amidror, The Theory of the Moiré Phenomenon, Kluwer, 2000, pages 249-352). Then, one replaces curvilinear lines of the line grating by bands, yielding a band grating. And finally, one verifies if the result is visually pleasing or not, and if not modifies the parameters of the base and revealing transformations and visualize again the results. When one of the layers layout is curvilinear, this trial and error method does not allow to compute a base band grating layer layout given a reference band moiré image layout and a revealing line grating layout. In addition, since the method relies on trial and error, it does not support the derivation of complicated geometric transformations, such as computing a base layer, which in superposition with a revealing layer forming a spiral shaped line grating yields a meaningful, visually pleasant band moiré image. The only reference band moiré image available with the trial and error method is the band moiré image produced by superposing the base and revealing layer derived thanks to the trial and error procedure.
Furthermore, U.S. patent application Ser. No. 10/270,546 (Hersch and Chosson) does neither give a precise technique for generating a reference rectilinear band moiré image layout with curvilinear base and revealing layer layouts nor does it give a means of generating a desired reference curvilinear band moiré image layout with a predetermined rectilinear or curvilinear revealing layer layout.
U.S. patent application Ser. No. 10/270,546 teaches a method for creating variations of the appearing moiré patterns when moving the revealing layer on top of the base layer, however these variations rely only on modifications of the shapes embedded within the base band layer and do not rely, as in the present disclosure, on the geometric transformations of the base layer and/or the revealing layer.
The present disclosure provides a band moiré image layout model allowing to compute not only the layout of a rectilinear band moiré image produced by superposing a rectilinear base band layer and a rectilinear revealing layer, but also in which direction and at which speed the rectilinear moiré shapes move when translating a the rectilinear revealing layer on top of the rectilinear base layer. For a curvilinear base layer and a curvilinear or rectilinear revealing layer, that model computes exactly the layout of the resulting rectilinear or curvilinear band moiré image obtained by superposing the base and revealing layers. Furthermore, one may specify a desired rectilinear or curvilinear band moiré image as well as one of the layers and the model is able to compute the layout of the other layer.
Let us also note that the properties of the moiré produced by the superposition of two line gratings are well known (see for example K. Patorski, The moiré Fringe Technique, Elsevier 1993, pp. 14-16). Moiré fringes (moiré lines) produced by the superposition of two line gratings (i.e. set of lines) are exploited for example for the authentication of banknotes as disclosed in U.S. Pat. No. 6,273,473, Self-verifying security documents, inventors Taylor et al.
Curved moiré fringes (moiré lines) produced by the superposition of curvilinear gratings are also known (see for example Oster G., Wasserman M., Zwerling C. Theoretical Interpretation of Moiré Patterns. Journal of the Optical Society of America, Vol. 54, No. 2, 1964, 169-175) and have been exploited for the protection of documents by a holographic security device (U.S. Pat. No. 5,694,229, issued Dec. 2, 1997, K. J. Drinkwater, B. W. Holmes).
In U.S. patent application Ser. No. 10/270,546 as well as in the present invention, instead of using a line grating as base layer, we use as base layer a band grating incorporating in each band an image made of one-dimensionally compressed original patterns of varying shapes, sizes, intensities and possibly colors. Instead of obtaining simple moiré fringes (moiré lines) when superposing the base layer and the revealing line grating, we obtain a band moiré image which is an enlarged and transformed instance of the original band image.
Joe Huck, a prepress professional, in his publication (2003) entitled “Mastering Moirés. Investigating Some of the Fascinating Properties of Interference Patterns, see also http://pages.sbcglobal.net/joehuck”, created band moiré images, both for artistic purposes and for creating designs incorporating moiré shapes floating within different perceived depth planes thanks to parallax effects. His publication only reports about vertically replicated horizontal base bands and a revealing layer made of horizontal lines, thereby generating moiré shapes moving only in the vertical direction. In contrast to the present invention, he neither provided a general-purpose framework for predicting the geometry of band moiré images as a function of base and revealing layer layouts, nor did he consider geometric transformations of base and revealing layers. In addition, he didn't consider applying band moiré images for document authentication.